The invention relates to a device for non-contact measurement of speed, displacement travel and/or distance of a reflecting measurement object, comprising a laser, in particular a semiconductor laser, as a light source, in which the level of light intensity in the laser resonator influences the optical length thereof, a transmitting and receiving optical system for directing laser light from the semiconductor laser onto the measurement object and for feeding back a part of the laser light reflected by the measurement object into the laser resonator, and a detector means for detecting and analysing the laser light which is emitted by the semiconductor laser and which is modulated in dependency on the laser light feedback from the measurement object into the laser resonator.
It is already known to use a semiconductor laser (laser diode) as a receiver or as a "converter" for the light which returns from the measurement object, which converts individual photons which are fed back from the object into the laser resonator, into changes in the laser emission parameters (for example intensity). By virtue of that conversion procedure, it is possible, even when dealing with very weak signals from the measurement object, to obtain the measurement information from relatively high levels of optical power (fractions of the laser output power), which makes only extremely low levels of requirement in regard to the detection system to be used. The article "Ranging and velocimetry signal generation in a backscatter-modulated laser diode" by Peter J. de Groot, Gregg M. Gallatin and Steven H. Macomber in Appl. Opt. 27, No 21, 1988, pages 4475-4480, describes such a device for non-contact distance measurement of a diffusely scattering measurement object. The transmitting and receiving optical system used in that arrangement can be of a very simple design configuration and does not require an expensive adjustment operation, in contrast to conventional interferometers for length measurement purposes. The known device makes use of the fact that the light which is fed back from the measurement object into the laser resonator results in amplitude modulation of the light which is emitted by the laser diode. For distance measurement purposes, the laser diode current and therewith the optical phase in modulated, and the procedure involves looking for amplitude modulation of the output signal of the laser diode at the frequency of that phase modulation. A serious disadvantage of the known device however is the fact that measurement of speed and/or distance is possible only up to a relatively short distance of the measurement object from the laser diode (a few meters when using typical laser diodes).